Caption: Hurricane formation. Illustration showing the circulation of air within a hurricane (tropical cyclone) after it has formed. The process starts with the rapid formation of clouds over warm tropical waters. This leads to updrafts and air moving inwards (red arrows). The winds and clouds then begin to rotate due to the effect of the Earth's rotation. As the energy within the storm builds, wind speeds increase with a hurricane declared when the winds reach 120 kilometres per hour. Structures seen in a mature hurricane include the eye, the eye wall, bands of rain, a spiralling vortex of warm air in the eye, and an outflow of cold air at the outer edges of the hurricane.